1. Field of the Invention
The invention has to do with electric arc welding, and especially with automatic pinpoint arc welding using filler metal under gas protection, currently called MAG spot welding.
2. Description of the Prior Art
This type of welding is generally used for assemblages of high resistance parts or parts for which conventional resistance welding cannot be envisaged because of the impossibility of reaching both surfaces of the assemblages at the same time.
This type of welding process essentially brings into operation the filler wire, fed by a reel placed on an unreeling device equipped with a brake and driven by a motor, and the welding torch which guides this wire up to the location of the weld, ensures electrical contact with this wire and distributes the protection gas.
This process presents numerous drawbacks, owing first of all to the fact that the different parts of the installation are spread around and connected by wire conveyor sheaths, which causes haphazard operation as a result of friction and clogging of the sheaths. Furthermore, the return of the welding current (negative pole) is generally accomplished by plugging into a ground external to the torch, often located far from it. Moreover, to insure proper welding, the sheets have to be perfectly laid out alongside each other and held tightly together during the welding, which more often than not is done by means of an external presser bar located some distance from the welding zone and adapted to each particular case. In addition, the reel brake, generally constituted by friction washers, delivers a very variable brake-gear torque per revolution over time, causing haphazard reeling. Since there is generally no end-of-wire pick-up and the welding current is kept on when the reels are empty, a surge in the arc current occurs, systematically deteriorating the contact tube of the torch. Finally, to maintain the position and quality of the weld point, the position of the welding torch must be constant, which is difficult to accomplish by means of the traditional anchorings which cause debris that add to the actual work debris of the torch, with all the above requiring readjustment every time the torch is disassembled.